From: Harry Spier <harryspier@...>
Date: 2010-09-14T11:35:09+09:00
Subject: Re: Converting from one class to another in Ruby


Dear Robert,

Many thanks for your reply.
I have simplified somewhat.  Its an OCR application where Binary_matrix 
is a black and white pixel representation of a page of text(or line of 
text, or letter or part of a letter etc.).  There are many different 
objects which are to be converted into Binary_matrix such as the above 
portions of an existing Binary_matrix object, or Rmagick ImageList 
objects, or graphic objects processed by other packages than RMagick. 
This list will definitely grow in the future so for that reason I wanted 
the conversion to Binary_matrix logic to reside in the class being 
converted to Binary_matrix not in the Binary_matrix class.

I.e. I didn't want a list of statements in Binary_matrix initialize 
method like:
if arg.class == ImageList then ImageList_conversion end
if arg.class == JPEG then jpeg_conversion end
etc. etc.
Rather I wanted my Binary_matrix object to check if the object to be 
converted to a Binary_matrix had a to_Binary_matrix method and then to 
call it.

I also require all the Array methods for Binary_matrix because of the 
complex traversing and manipulation of the pixel data.  To give a 
trivial example: does a black pixel in line n touch a black pixel in 
line n+1? What are the array ranges of a contiguous group of black 
pixels etc. Change a contiguous range of pixels to white (i.e. clear up 
noise) etc.

Again because of complexity of the application I wanted to make the code 
as humanly readable as possible.  So I want to be able to say
page_in_pixels = page_image.to_Binary_matrix
as well as
page_in_pixels = Binary_matrix(page_image)
(I know that may sound trivial but when you go back to look at your code 
a few months later, everything that makes it more easily readable 
helps.)
So for those reasons the solution I came up with is as follows:

(I'm not saying its necessarily a good solution and I really wish I 
could make it less clunky. if there was some way to tell whether method 
to_Binary_matrix was being called within the context of Binary_matrix 
initialize method or outside it then the flag -from_initialize_method- 
wouldn't be necessary)

class A
   def to_Binary_matrix(from_initialize_method = false)
      if from_initialize_method
         return self
        else
         return Binary_matrix.new(self)
      end
    end
...
end

class Binary_matrix < Array
   def initialize(arg)
     called_from_initialize =true
     if arg.respond_to? :to_Binary_matrix
       arg = arg.to_Binary_matrix (called_from_initialize)
     else
      ....do some error reporting
     end
     validate arg #Is the Binary_matrix object correctly formed?
   end
...
end
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